The present invention relates to a method of producing a diaphragm of an acoustic instrument, having a low density and a high elasticity. More particularly, the invention is concerned with a method of easily producing the diaphragm of an acoustic instrument, the method including blending and kneading plastic and carbon powders with each other, shaping the blend, and carbonizing the shaped blend by heating.
Generally speaking, the diaphragms of acoustic instruments, particularly the diaphragm of a speaker is required to have light weight, large rigidity and a large ratio E/.rho. of Young's modulus E to density .rho., so that it may reproduce the acoustic signal efficiently over a wide range of frequency and at a high fidelity.
For this reason, conventionally, wood pulps, plastics, aluminum, titanium and the like materials have been used as the material of the diaphragm. These conventional materials, however, could not fully meet the above requirements.
Also, it has been proposed and actually carried out to make use of carbon materials. One of these carbon materials is a composite material of carbon fibers and a plastic. This composite material, however, cannot provide sufficient rigidity, when it is formed into a tabular forms of diaphragm, partly because of insufficient binding of carbon fibers attributable to the lubricating nature of the surface of carbon fiber itself, and partly because of the large anisotropy of the carbon fibers.
Under these circumstances, the present inventors have proposed a diaphragm composed of carbonized or graphitized plastic, so as to make the most of the advantages of carbon as the diaphragm material, i.e. light weight, high rigidity and large ratio of Young's modulus E to the density .rho..
It is difficult, however, to carbonize or graphitize the plastic while preserving the shape of the diaphragm. At the same time, a high orientation which would ensure a high elasticity cannot be obtained unless a suitable tension is applied to the diaphragm material. In addition, the diaphragm material inconveniently exhibits a large distortion in the course of carbonization or graphitization, resulting in cracking of the diaphragm.